Light beam scanning apparatus

ABSTRACT

A light beam position detecting device detects the passage positions of a plurality of light beams for scanning the surface of a photosensitive drum. The light beam position detecting device has a sensor pattern for generating an output which is continuously changed in a wide range with a variation in the passage position of the light beam in a sub-scanning direction perpendicular to a main scanning direction of the light beam. The sensor pattern precisely detects the relative scanning position of the light beam in a wide range. The passage positions of the plurality of light beams for scanning the surface of the photosensitive drum are controlled to a preset position based on the output of the sensor pattern.

The present application is a divisional of U.S. application Ser. No.10/067,868, filed Feb. 8, 2002, which claims priority of U.S. patentapplication Ser. No. 09/461,210, filed Dec. 15, 1999 (now U.S. Pat. No.6,496,212), which claim priority of Japanese Patent Application10-356022, filed Dec. 15, 1998, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates to an image forming apparatus such as a digitalcopying machine or laser printer for scanning and exposing a singlelight beam or a plurality of light beams emitted from a semiconductorlaser on a photosensitive drum surface to form a single electrostaticlatent image on the photosensitive drum and more particularly to a lightbeam scanning apparatus provided on the image forming apparatus forscanning the single light beam or the plurality of light beams.

In recent years, various types of digital copying machines for formingan image by scanning and exposing the light beam and using theelectrophotographic process are developed. Recently, in order to furtherenhance the image forming speed, a multi-beam type digital copyingmachine in which a plurality of light beams are emitted tosimultaneously scan a plurality of lines by use of the plurality oflight beams has been developed.

The multi-beam type digital copying machine includes an optical systemunit as a light beam scanning apparatus having a plurality ofsemiconductor laser oscillators for emitting light beams, a polygonalrotating mirror such as a polygon mirror for reflecting the light beamsoutput from the plurality of semiconductor laser oscillators towards thephotosensitive drum and scanning the photosensitive drum by the lightbeams and a collimator lens and f-θ lens as main components.

Conventionally, in the above multi-beam type digital copying machine,control of the exposure position of the light beam in the main scanningdirection and control of the exposure position in the sub-the mainscanning direction (that is, control of the passage position of thelight beam) are effected in the optical system unit in order to form animage with high image quality. A concrete example of the above techniqueis disclosed in, for example, Japanese Pat. Appln. KOKOKU PublicationNo. 1-43294, Japanese Pat. Appln. KOKOKU Publication No. 3-57452,Japanese Pat. Appln. KOKOKU Publication No. 3-57453, Japanese UM. Appln.KOKOKU Publication No. 5-32824, Japanese Pat. Appln. KOKAI PublicationNo. 7-72399, Japanese Pat. Appln. KOKAI Publication No. 7-228000,Japanese Pat. Appln. KOKAI Publication No. 9-210849, Japanese Pat.Appln. KOKAI Publication No. 9-258125, Japanese Pat. Appln. KOKAIPublication No. 9-314901 and Japanese Pat. Appln. KOKAI Publication No.10-76704. However, the techniques disclosed in the above publicationshave the following problems.

That is, for control of the light beam exposure position in the mainscanning direction, it is important to mount a light beam detectingdevice constructed by a plurality of optical sensors in a presetdirection with respect to the main scanning direction of the light beam.That is, if the light beam detecting device is mounted in an inclinedstate, it becomes impossible to correctly detect the light beam positionin the main scanning direction and, for example, there occurs a problemthat a vertical line cannot be drawn straight.

However, an example indicating that the sensor itself has a function ofdetecting the relation between the mounting direction of the light beamdetecting device and the main scanning direction of the light beam isdisclosed only in Japanese Pat. Appln. KOKAI Publication No. 9-314901.Even in this example, the inclination detecting range is extremelynarrow and there occurs a problem that detection and adjustment of thelight beam position are difficult.

For control of the light beam position in the sub-scanning direction,examples in which the passage position of the light beam in thesub-scanning direction is replaced by time at which the light beampasses the sensor and detected are disclosed in Japanese Pat. Appln.KOKAI Publication No. 7-72399, Japanese Pat. Appln. KOKAI PublicationNo. 7-228000 and Japanese Pat. Appln. KOKAI Publication No. 9-210849.

However, if a variation occurs in the f-θ. characteristic of the f-θlens mounted on the optical system unit or a variation occurs in therotation speed of the polygon mirror, then a variation will occur in thescanning speed of the light beam on the sensor and a detection error mayoccur when the detection method based on the passage time of the lightbeam is used.

Further, in Japanese Pat. Appln. KOKAI Publication No. 9-258125,Japanese Pat. Appln. KOKAI Publication No. 9-314901 and Japanese Pat.Appln. KOKAI Publication No. 10-76704, examples in which the passageposition of the light beam is driven into a portion between specifiedsensor patterns formed on the light beam detecting device to set thepassage position of the light beam in a preset position are shown.However, with this construction, it is necessary to independently drivethe light beams to the preset passage position and actuators forcontrolling the passage positions of the light beams are required by anumber corresponding to the number of light beams. That is, incomparison with a case wherein one light beam is used as a reference andthe passage positions of the remaining light beams are controlled, thenumber of actuators is larger by one and the cost becomes higher.

Further, if the detecting pattern for driving the light beam to thepreset position is used, the precision of detection is high, but a range(detection range) in which each sensor output of the detecting patternvaries with a variation in the passage position of the light beam isnarrow. Therefore, the control process becomes complicated and time forthe control process becomes long.

If it is possible to control the passage position of each light beam fora plurality of resolutions, the number of sensor patterns for drivingeach light beam is increased and the structure of the sensor becomescomplicated.

BRIEF SUMMARY OF THE INVENTION

A first object of this invention is to provide a light beam scanningapparatus capable of enlarging the range (detection range) in which onesensor can respond to a variation in the passage position of a lightbeam, simplifying the control process and enhancing the controloperation speed.

A second object of this invention is to provide a light beam scanningapparatus in which the number of actuators such as galvanomirrors forcontrolling the passage positions of the light beams is suppressed.

A third object of this invention is to provide a light beam scanningapparatus capable of coping with a plurality of resolutions with thesimple sensor construction.

A fourth object of this invention is to provide a light beam scanningapparatus having a sensor for detecting the mounting inclination of thelight beam detecting device with respect to the main scanning directionof the light beam in a wide range in the light beam detecting device.

A fifth object of this invention is to provide a light beam scanningapparatus capable of precisely detecting the passage position of thelight beam irrespective of the scanning speed of the light beam on thesensor.

In order to achieve the above objects, according to one aspect of thepresent invention, there is provided a light beam scanning apparatuscomprising: light beam emitting means for outputting a light beam; abeam scanner for reflecting the light beam output from the light beamemitting device towards a to-be-scanned surface to scan theto-be-scanned surface by use of the light beam in a main scanningdirection; a first beam position detector for detecting the light beamscanned on the to-be-scanned surface by the beam scanner and generatingan analog signal which is continuously changed with a variation in thepassage position in a sub-scanning direction perpendicular to the mainscanning direction of the light beam; and controller for controlling theposition of the light beam scanned by the beam scanner on theto-be-scanned surface to a preset position based on the result ofdetection of the first beam position detector.

Further, according to this invention, a plurality of light beam emittingdevices are provided and the beam scanner scans the to-be-scannedsurface by use of a plurality of light beams emitted from the pluralityof light beam emitting devices. The scanning apparatus further compriseslight beam passage position changing means of a number smaller than thenumber of the plurality of light beam emitting devices by one, forchanging the passage position of the light beam in the sub-scanningdirection. The controller determines one of the plurality of light beamsas a reference beam and changing the relative passage position of theremaining light beams with respect to the passage position of thereference light beam by use of the light beam passage position changingmeans.

Therefore, the number of actuators such as galvanomirrors forcontrolling the passage positions of the light beams can be suppressed.Further, the relative passage positions of the plurality of light beamscan be precisely detected irrespective of the scanning speed of thelight beam on the sensor.

According to another aspect of the present invention, there is provideda light beam scanning apparatus comprising: a plurality of light beamemitting devices for outputting light beams; a beam scanner forreflecting the light beams output from the light beam emitting devicestowards a to-be-scanned surface to scan the to-be-scanned surface by useof the light beams in a main scanning direction; a first beam positiondetector for detecting the light beam scanned on the to-be-scannedsurface by the beam scanner and generating an analog signal which iscontinuously changed with a variation in the passage position in asub-scanning direction perpendicular to the main scanning direction ofthe light beam; a first target light detecting member having a firstpassage target and disposed separately from the first beam positiondetector in the main scanning direction; a second target light detectingmember having a second passage target separated from the first passagetarget in the sub-scanning direction by a distance corresponding topreset resolution; light beam passage position changing means forchanging the passage position of at least one of the plurality of lightbeams; and a controller for controlling the relation of the respectivepassage positions of the plurality of light beams to a preset relationby use of the light beam passage position changing means based on theoutputs of the first beam position detector respectively obtained whenthe light beam has passed through the first and second passage targets.

There is further provided a light beam scanning apparatus the above,wherein the controller includes: calculating means for calculating adifference between the outputs of the first beam position detectorrespectively obtained when the light beam has passed through the firstand second passage targets; and means for changing the passage positionof one of first and second light beams among the plurality of lightbeams by use of the first beam passage position changing means to setthe difference calculated by the calculating means equal to a differencebetween outputs of the beam position detector respectively obtained atthe time of scanning by the first and second light beams.

There is further provided a light beam scanning apparatus the above,wherein the first beam position detector includes second and third beamposition detectors; the second beam position detector generates anoutput which continuously decreases with a variation in the passageposition of the light beam in the sub-scanning direction, the third beamposition detector is disposed separately from the second beam positiondetector in the sub-scanning direction and generates an output whichcontinuously increases with a variation in the passage position of thelight beam, and the controller controls the passage position of thelight beam to a preset position based on the results of detection of thesecond and third beam position detectors.

There is further provided a light beam scanning apparatus the above,further comprising: a fifth beam position detector disposed separatelyfrom the second and third beam position detectors in the main scanningdirection, for detecting the light beam used for scanning theto-be-scanned surface by the beam scanner and generating an output whichcontinuously decreases with a variation in the passage position of thelight beam; a sixth beam position detector disposed adjacent to thefifth beam position detector in the sub-scanning direction, fordetecting the light beam used for scanning the to-be-scanned surface bythe beam scanner and generating an output which continuously increaseswith a variation in the passage position of the light beam; andinclination detecting means for detecting whole inclinations of thesecond to sixth beam position detectors with respect to the scanningdirection of the light beam based on the results of detection of thesecond, third, fifth and sixth beam position detectors.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 is a structural view schematically showing the structure of adigital copying machine according to an embodiment of this invention;

FIG. 2 is a view showing the positional relation between the structureof the optical system unit and the photosensitive drum;

FIG. 3 is a block diagram showing the control system including anoptical system control section as a main portion;

FIG. 4 is a structural view schematically showing the structure of alight beam detecting device;

FIG. 5 is a block diagram showing the control system for illustrating amethod for extracting passage position information of the light beam andcontrolling the galvanomirror based on an output from the light beamdetecting device shown in FIG. 5;

FIGS. 6A to 6C are diagram showing the relation between the passageposition of the light beam, outputs of the sensor patterns of the lightbeam detecting device, an output of a differential amplifier, and anoutput of an integrator;

FIG. 7 is a graph showing the relation between the passage position ofthe light beam and an output voltage of the integrator;

FIG. 8 is a graph for illustrating a method for detecting the passageposition of the light beam with high resolution set for a necessaryportion;

FIG. 9 is a structural view schematically showing another example of thestructure of the light beam detecting device;

FIG. 10 is a structural view schematically showing still another exampleof the structure of the light beam detecting device;

FIG. 11 is a structural view schematically showing another example ofthe structure of the light beam detecting device;

FIG. 12 is a structural view schematically showing another example ofthe structure of the light beam detecting device;

FIG. 13 is a block diagram of a control system for illustrating thelight beam passage position control process using the light beamdetecting device of FIG. 12;

FIG. 14 is a diagram for illustrating the relation between the positionof the sensor pattern in the light beam detecting device of FIG. 12 andan output thereof;

FIG. 15 is a structural view schematically showing still another exampleof the structure of the light beam detecting device;

FIGS. 16A and 16B are views for illustrating the sizes of sensorpatterns in the light beam detecting device of FIG. 15;

FIG. 17 is a flowchart for illustrating a method for making it possibleto detect both of the absolute light beam passage position and relativepositional relation by use of the light beam detecting device of FIG.15;

FIG. 18 is a structural view schematically showing another example ofthe structure of the light beam detecting device;

FIG. 19 is a view for illustrating the size of sensor pattern in thelight beam detecting device of FIG. 18;

FIG. 20 is a flowchart for illustrating the light beam passage positioncontrol process using the light beam detecting device of FIG. 18;

FIG. 21 is a structural view schematically showing still another exampleof the structure of the light beam detecting device;

FIGS. 22A and 22B are views for illustrating the sizes of sensorpatterns in the light beam detecting device of FIG. 21; and

FIG. 23 is a structural view schematically showing another example ofthe structure of the light beam detecting device.

DETAILED DESCRIPTION OF THE INVENTION

There will now be described an embodiment of this invention withreference to the accompanying drawings.

FIG. 1 schematically shows the structure of a digital copying machineused as an image forming apparatus to which a light beam scanningapparatus according to an embodiment of this invention is applied. Thatis, the digital copying machine includes, for example, a scanner section1 used as image reading means and a printer section 2 used as imageforming means. The scanner section 1 includes a first carriage 3 andsecond carriage 4 movable in a direction indicated by an arrow in FIG.1, image forming lens 5 and photoelectric conversion device 6.

In FIG. 1, an original O is placed on an original table 7 formed oftransparent glass with the front surface down and the front right sidein the short-side direction of the original table 7 is set as the centerreference for the reference of placement of the original O. The originalO is pressed on the original table 7 by an original fixing cover 8freely set in the open/closed state.

The original O is illuminated by a light source 9 and reflected lightfrom the original is converged onto the light receiving surface of thephotoelectric conversion device 6 via mirrors 10, 11, 12 and imageforming lens 5. The first carriage 3 having the light source 9 andmirror 10 mounted thereon and the second carriage 4 having the mirrors11, 12 mounted thereon are moved in a relative speed of 2:1 to set thelength of the optical path constant. The first carriage 3 and secondcarriage 4 are moved from the right side to the left side in synchronismwith a read timing signal by a carriage driving motor (not shown).

The image of the original O thus placed on the original table 7 issequentially read for each line by the scanner section 1 and a readoutput is converted into an 8-bit digital image signal indicating thedensity of the image in an image processing apparatus (not shown).

The printer section 2 includes an image forming section 14 having acombination of an optical system unit 13 and an electrophotographicsystem capable of forming the image on paper P which is an image formingmedium. That is, an image signal read from the original O by the scannersection 1 is converted to a light beam (which is hereinafter simplyreferred to as a light beam) from the semiconductor laser oscillatorafter it is processed in the image processing section (not shown). Inthis embodiment, a multi-beam optical system using a plurality of (twoor more) semiconductor laser oscillators is used.

The construction of the optical system unit 13 is explained later indetail, but a plurality of semiconductor laser oscillators provided inthe unit effect the light emitting operation according to a lasermodulation signal output from the image processing section (not shown)and a plurality of light beams emitted from the laser oscillators arereflected by the polygon mirror, used as scanning lights and output tothe exterior of the unit.

A plurality of light beams emitted from the optical system unit 13 areconverged as the scanning light of a spot having necessary resolution ona point of the exposing position X on the photosensitive drum used as animage carrier and scanned for exposure. As a result, an electrostaticlatent image corresponding to an image signal is formed on thephotosensitive drum 15.

An electric charger 16 for charging the surface of the photosensitivedrum 15, developing unit 17, transfer charger 18, separation charger 19,cleaner 20 and the like are arranged around the photosensitive drum 15.The photosensitive drum 15 is rotated at a preset circumferential speedby a driving motor (not shown) and charged by the electric charger 16set to face the surface thereof. A plurality of light beams (scanninglight) are converged in a spot form on the point of exposure position Xon the charged photosensitive drum 15.

The electrostatic latent image formed on the photosensitive drum isdeveloped by use of toner (developing agent) from the developing unit17. The toner image formed on the photosensitive drum 15 by thedevelopment process is transferred to paper P supplied at adequatetiming by the paper feeding system on a point of the transferringposition by the transfer charger 18.

The paper feeding system separately feeds sheets of paper P in the paperfeeding cassette 21 provided in the bottom portion for each sheet by useof a paper feeding roller 22 and separation roller 23. Then, the paperis fed to a resist roller 24 and supplied to the transfer position atpreset timing. On the downstream side with respect to the transfercharger 18, a paper feeding mechanism 25, fixing unit 26 and a paperdischarging roller 27 for discharging paper P which has been subjectedto the image forming process are provided. With this construction, thetoner image on the paper P on which the toner image has been transferredis fixed by the fixing unit 26 and then the paper P is discharged to anexternal paper discharging tray 28 via the paper discharging roller 27.

The remaining toner on the surface of the photosensitive drum 15 fromwhich the image has been transferred to the paper P is removed by thecleaner 20, and it is restored into the initial state and set into thestandby state for formation of a next image.

The above operation is repeatedly effected to continuously effect theimage forming operation.

As described above, the original 0 placed on the original table 7 isread by the scanner section 1 and information read by the scannersection is subjected to a series of processes in the printer section 2and then recorded on the paper P as a toner image.

Next, the optical system unit 13 is explained.

FIG. 2 shows the positional relation between the structure of theoptical system unit 13 and the photosensitive drum 15. The opticalsystem unit 13 contains semiconductor laser oscillators 31 a, 31 b, 31c, 31 d as four light beam emitting means, for example, and thehigh-speed image forming process can be attained without extremelyenhancing the rotation speed of the polygon mirror by causing thesemiconductor laser oscillators 31 a, 31 b, 31 c, 31 d to simultaneouslyeffect image formation for the respective scanning lines.

That is, the laser oscillator 31 a is driven by a laser driver 32 a, anda light beam output therefrom passes through half mirrors 34 a, 34 bafter passing through a collimator lens (not shown) and is made incidenton a polygon mirror 35 used as a polygonal rotating mirror.

The polygon mirror 35 is rotated at a constant speed by a polygon motor36 driven by a polygon motor driver 37. Thus, reflected light from thepolygon mirror 35 scans in a preset direction at an angular speeddetermined by the rotation speed of the polygon mirror 36. The lightbeam scanned by the polygon mirror 35 passes through an f-θ lens (notshown) and scans the surface of the photosensitive drum 15 and the lightreceiving surface of a light beam detecting device 38 acting as lightbeam power detecting means, light beam passage timing detecting meansand light beam position detecting means at a constant speed according tothe f-θ characteristic of the f-θ lens.

The laser oscillator 31 b is driven by a laser driver 32 b and a lightbeam output therefrom is reflected by a galvanomirror 33 b after passingthrough a collimator lens (not shown) and is further reflected by a halfmirror 34 a. The reflected light from the half mirror 34 a passesthrough a half mirror 34 b and is made incident on the polygon mirror35. The path along which the light beam is transmitted after beingreflected by the polygon mirror 35 is the same as in the case of thelight beam emitted from the laser oscillator 31 a and it passes throughthe f-θ lens (not shown) and scans the surface of the photosensitivedrum 15 and the light receiving surface of the light beam detectingdevice 38 at a constant speed.

The laser oscillator 31 c is driven by a laser driver 32 c and a lightbeam output therefrom is reflected by a galvanomirror 33 c after passingthrough a collimator lens (not shown), passes through a half mirror 34 cand is reflected by a half mirror 34 b and made incident on the polygonmirror 35.

The path along which the reflected light is transmitted after beingreflected by the polygon mirror 35 is the same as in the case of thelight beams emitted from the laser oscillators 31 a, 31 b and it passesthrough the f-θ lens (not shown) and scans the surface of thephotosensitive drum 15 and the light receiving surface of the light beamdetecting device 38 at a constant speed.

The laser oscillator 31 d is driven by a laser driver 32 d and a lightbeam output therefrom is reflected by a galvanomirror 33 d after passingthrough a collimator lens (not shown) and is reflected by a half mirror34 c, reflected by the half mirror 34 b and made incident on the polygonmirror 35. The path along which the reflected light is transmitted afterbeing reflected by the polygon mirror 35 is the same as in the case ofthe light beams emitted from the laser oscillators 31 a, 31 b, 31 c andit passes through the f-θ lens (not shown) and scans the surface of thephotosensitive drum 15 and the light receiving surface of the light beamdetecting device 38 at a constant speed.

The laser drivers 32 a to 32 d contain automatic power control (APC)circuits and drive the laser oscillators 31 a to 31 d to always emitlights at light emission power levels set by a main control section(CPU) 51 as will be described later.

Light beams thus output from the different laser oscillators 31 a, 31 b,31 c, 31 d are combined by the half mirrors 34 a, 34 b, 34 c and fourlight beams are transmitted towards the polygon mirror 35.

Therefore, the four light beams can simultaneously scan the surface ofthe photosensitive drum 15 and record an image at speed four times thatof the conventional single beam case if the rotation speed of thepolygon mirror 35 is the same.

The galvanomirrors 33 b, 33 c, 33 d adjust (control) the positionalrelation of the light beams output from the laser oscillators 31 b, 31c, 31 d in the sub-scanning direction with respect to the light beamoutput from the laser oscillator 31 a and galvanomirror driver circuits39 b, 39 c, 39 d for driving the galvanomirrors are respectivelyconnected thereto.

Further, light beam detecting device adjusting motors 38 a, 38 b foradjusting the mounting position of the light beam detecting device 38and the inclination thereof with respect to the scanning direction ofthe light beam are mounted on the light beam detecting device 38.

The light beam detecting device 38 detects the passage positions,passage timings and powers of the four light beams and is arranged nearthe end portion of the photosensitive drum 15 such that the lightreceiving surface thereof can be set in flush with the surface of thephotosensitive drum 15. Control of the galvanomirrors 33 b, 33 c, 33 dfor the respective light beams (control of the image forming position inthe sub-scanning direction), control of light emission powers(intensity) of the laser oscillators 31 a, 31 b, 31 c and control of thelight emission timings thereof (control of the image forming position inthe main scanning direction) are effected based on the detection signalfrom the light beam detecting device 38 (the control processes areexplained later in detail). The beam detecting device 38 generatesanalog signals according to the result of the detection. In order toform analog signals to effect the control processes, a light beamdetecting device output processing circuit 40 is connected to the lightbeam detecting device 38.

Next, the control system is explained. FIG. 3 shows the control systemmainly for controlling the multi-beam optical system. That is, 51denotes a main control section for controlling the whole portion whichis constructed by a CPU, for example, and to which a memory 52, controlpanel 53, external communication interface (I/F) 54, laser drivers 32 a,32 b, 32 c, 32 d, polygon mirror motor driver 37, galvanomirror drivingcircuits 39 b, 39 c, 39 d, light beam detecting device output processingcircuit 40, synchronizing circuit 55 and image data interface (I/F) 56are connected.

The image data I/F 56 is connected to the synchronizing circuit 55 andthe image processing section 57 and page memory 58 are connected to theimage data I/F 56. The scanner section 1 is connected to the imageprocessing section 57 and the external interface (I/F) 59 is connectedto the page memory 58.

Now, flow of image data when the image is formed is briefly explainedbelow.

First, in the case of copying operation, an image of the original 0 seton the original table 7 is read by the scanner section 1 and fed to theimage processing section 57 as described before. The image processingsection 57 subjects the image signal from the scanner section 1 to theknown shading correction process, various filtering processes, gradationprocess, gamma process and the like, for example.

Image data from the image processing section 57 is supplied to the imagedata I/F 56. The image data I/F 56 plays a role of distributing theimage data to the four laser drivers 32 a, 32 b, 32 c, 32 d.

The synchronizing circuit 55 generates a clock synchronized with thetiming at which each light beam passes on the light beam detectingdevice 38 and transmits image data as laser modulation signals from theimage data I/F 56 to the respective laser drivers 32 a, 32 b, 32 c, 32 din synchronism with the clock.

Thus, image formation synchronized in the main scanning direction (inthe correct position) can be effected by transferring image data insynchronism with scanning of the respective light beams.

Further, the synchronizing circuit 55 includes a sample timer forforcedly causing the laser oscillators 31 a, 31 b, 31 c, 31 d to emitlights in a non-image area and controlling the powers of the light beamsand a logic circuit for causing the laser oscillators 31 a, 31 b, 31 c,31 d to emit lights on the light beam detecting device 38 in an order ofthe light beams in order to determine image forming timings of therespective light beams.

The control panel 53 is a man-machine interface for starting the copyingoperation and setting the number of sheets, for example.

The digital copying machine is constructed to effect not only thecopying operation but also the image forming and outputting operationfor forming and outputting image data input from the exterior via theexternal I/F 59 connected to the page memory 58. Image data input viathe external I/F 59 is temporarily stored in the page memory 59 and thentransmitted to the synchronizing circuit 55 via the image data I/F 56.

Further, if the digital copying machine is controlled from the exteriorvia a network, for example, the external communication I/F 54 plays arole of the control panel 53.

The galvanomirror driving circuits 39 b, 39 c, 39 d are circuits fordriving the galvanomirrors 33 b, 33 c, 33 d according to instructionvalues from the main control section 51. Therefore, the main controlsection 51 can freely control the angles of the galvanomirrors 33 b, 33c, 33 d via the galvanomirror driving circuits 39 b, 39 c, 39 d.

The polygon mirror driver 37 is a driver for driving the polygon motor36 for rotating the polygon mirror 35 which scans the four light beamsdescribed before. The main control section 51 can cause the polygonmotor driver 37 to start or stop the rotation and switch the rotationspeed. Switching of the rotation speed can be effected in a case wherethe rotation speed is made lower than a preset rotation speed asrequired and a recording resolution is changed when the passage positionof the light beam is confirmed by the light beam detecting device 38.

The laser drivers 32 a, 32 b, 32 c, 32 d have a function of forcedlycausing the laser oscillators 31 a, 31 b, 31 c, 31 d to emit lights inresponse to a forced light emission signal from the main control section51 irrespective of image data in addition to a function of causing thelaser lights to be emitted according to the laser modulation signalsynchronized with scanning of the light beam from the synchronizingcircuit 55 as described before.

Further, the main control section 51 sets powers of light beams emittedfrom the laser oscillators 31 a, 31 b, 31 c, 31 d for the laser drivers32 a, 32 b, 32 c, 32 d. The set value of the light emission power ischanged according to a variation in the process condition and detectionof the passage position of the light beam.

The memory 52 is to store information necessary for control. Forexample, the optical system unit 13 can be set into a state in whichimage formation can be instantly effected after turning ON the powersupply by storing, for example, the control amount of each of thegalvanomirrors 33 b, 33 c, 33 d, the circuit characteristic (offsetvalue of the amplifier) for detecting the passage position of the lightbeam, the order of incoming of the light beams and the like.

Next, the light beam detecting device 38 is explained.

FIG. 4 is a view showing the relation between the structure of the lightbeam detecting device 38 and the scanning direction of the light beam.Light beams from the four semiconductor laser oscillators 31 a, 31 b, 31c, 31 d are scanned in the main scanning direction from the right to theleft in the drawing by rotation of the polygon mirror 35 to cross thelight beam detecting device 38.

The light beam detecting device 38 includes two sensor patterns S1, S2which are long in the vertical direction, a sensor pattern S0 disposedbetween the two sensor patterns S1 and S2 and a holding base plate 38 afor integrally holding the sensor patterns S1, S0, S2.

The sensor pattern S1 is a pattern for detecting passage of the lightbeam to generate a reset signal (integration starting signal) of anintegrator as will be described later and the sensor pattern S2 is apattern for detecting passage of the light beam to generate a conversionstarting signal of an A/D converter as will be described later. Thesensor pattern S0 is a pattern for detecting the passage position of thelight beam and is formed to generate an output which is continuouslychanged with a variation in the passage position of the light beam.

As shown in FIG. 4, the sensor pattern S0 has such a shape that thedistance by which the light beam crosses the sensor pattern S0 becomeslonger as the passage position of the light beam becomes nearer to theupper side in FIG. 4. That is, if passage positions P1, P2, P3 of thelight beams are taken as an example, the distances by which the lightbeams cross the sensor pattern S0 are D1, D2, D3 and the relation ofD1>D2>D3 is obtained. Therefore, the period of signal outputting time ofthe sensor pattern S0 is changed according to the position through whichthe light beam passes.

For example, the sensor patterns S1, S2 are formed of photodiodes andintegrally formed on the holding base plate 38 a.

FIG. 5 is a block diagram showing the construction of a device forextracting light beam passage position information based on outputs fromthe sensor patterns S1, S0, S2 shown in FIG. 4 and controlling thegalvanomirrors.

As described before, pulse-form signals indicating that the light beamshave passed are output from the sensor patterns S1, S0, S2.

Further, a signal whose period of output time is changed with avariation in the light beam passage position (position in thesub-scanning direction) is output from the sensor pattern S0.

An output signal of the sensor pattern S0 is input to the non-invertinginput terminal (+) of a differential amplifier 60. The inverting inputterminal (−) of the differential amplifier 60 is supplied with an outputof a D/A converter 61. The amplification factor of the differentialamplifier 60 can be set by the main control section (CPU) 51.

The D/A converter 61 converts a digital signal from the main controlsection 51 into an analog signal. That is, the differential amplifier 60amplifies a difference between a set value input from the main controlsection 51 via the D/A converter 61 and an output of the sensor patternS0 with an amplification factor set by the main control section 51.

An output signal of the differential amplifier 60 is input to andintegrated by an integrator 42 used as integrating means. A pulse-formsignal output from the sensor pattern S1 is also input to the integrator42.

The pulse-form signal from the sensor pattern S1 is used as a resetsignal (integration starting signal) for resetting the integrator 42,and at the same time, starting the new integrating operation. Therefore,the integrator 42 is reset and starts to newly integrate the outputsignal from the sensor pattern S0 when the light beam passes on orcrosses the sensor pattern S1.

An output signal of the integrator 42 is input to an A/D converter 43used as converting means. A pulse-form signal output from the sensorpattern S2 is also input to the A/D converter 43. The A/D converter 43is triggered by the pulse-form signal output from the sensor pattern S2to A/D convert the output signal of the integrator 42.

That is, the A/D converter 43 converts the output signal of theintegrator 42 into digital data when the light beam reaches the sensorpattern S2 after passing on the sensor pattern S0 and supplies thedigital data to the main control section 51. When the A/D convertingoperation is terminated, the A/D converter 43 outputs an interruptsignal (INT) indicating that the A/D converting operation is terminatedto the main control section 51.

When receiving the interrupt signal from the A/D converter 43, the maincontrol section 51 reads the output of the A/D converter 43 to obtainthe newest light beam passage position information.

Then, the main control section 51 calculates the control amounts of thegalvanomirrors 33 b, 33 c, 33 d based on the thus obtained light beampassage position information, stores the result of calculation into thememory 52 if necessary and supplies the result of calculation to thegalvanomirror driving circuits 39 b, 39 c, 39 d.

As shown in FIG. 5, latches 44 b, 44 c, 44 d for holding data of theresult of calculation are provided in the galvanomirror driving circuits39 b, 39 c, 39 d. If the latches 44 b, 44 c, 44 d fetch data from themain control section 51, they hold the data until the data is updated.

Data items held in the latches 44 b, 44 c, 44 d are converted intoanalog signals (voltages) by D/A converters 45 b, 45 c, 45 d and inputto galvanomirror drivers 46 b, 46 c, 46 d. The drivers 46 b, 46 c, 46 ddrive the galvanomirrors 33 b, 33 c, 33 d according to the voltagesignals input from the D/A converters 45 b, 45 c, 45 d.

Therefore, in this embodiment, the light beam passage positions can becontrolled by operating the semiconductor laser oscillators for emittinglight beams whose passage positions are desired to be controlled,reading the output of the A/D converter 43 and controlling thegalvanomirrors 33 b, 33 c, 33 d based on read information.

Next, the operation of each section, that is, the state in which thelight beam passage position information is extracted when the light beampasses through the passage positions P1, P2, P3 is explained withreference to FIG. 6. In this case, in order to clarify the explanation,FIG. 6 shows a case wherein a set value into the D/A converter 61 is“0”. By setting the set value in the D/A converter 61 to “0”, thedifferential amplifier 60 can be dealt with as a simple amplifier. Therole of the D/A converter 61 is explained later.

FIG. 6A shows the operation when the light beam passes through theposition P1, FIG. 6B shows the operation when the light beam passesthrough the position P2 and FIG. 6C shows the operation when the lightbeam passes through the position P3.

When the light beam passes on the sensor pattern S1, the sensor patternS1 outputs a pulse-form signal, the integrator 42 is reset in responseto the pulse-form signal as shown in FIG. 6A and the output thereof isset to “0”. When the light beam reaches the sensor pattern S0, an outputsignal is generated from the sensor pattern S0 and a signal obtained byamplifying the output signal is output from the differential amplifier60.

As shown in FIG. 6A, in the case of the light beam P1, the differentialamplifier 60 outputs a positive signal in a period of time T1. Theintegrator 42 integrates the output signal and outputs an output voltageV1. As shown in FIG. 6B, in the case of the light beam P2, thedifferential amplifier 60 outputs a positive signal in a period of timeT2. As shown in FIG. 6C, in the case of the light beam P3, thedifferential amplifier 60 outputs a positive signal in a period of timeT3. Therefore, the output voltages of the integrator 42 for the lightbeams P2 and P3 are set to voltages V2 and V3 corresponding to theintegration periods of time.

As described before, the time periods T1 to T3 during which the lightbeam passes on the sensor pattern S0 are different depending on theposition (P1, P2, P3) in which the light beam passes on the sensorpattern S0. Since the passage time periods have the relation ofT1>T2>T3, the output voltages V1 to V3 of the integrator 42corresponding to the respective light beams have the relation ofV1>V2>V3.

Further, when the light beam passes on the sensor pattern S2, apulse-form signal is output from the sensor pattern S2 and the A/Dconverter 43 converts the voltage values V1 to V3 to correspondingdigital values.

The main control section 51 can roughly detect the position where thelight passes on the sensor pattern S0 by reading the digital valuesoutput from the A/D converter 43.

FIG. 7 is a graph showing the relation between the light beam passageposition and the output voltage of the integrator 42 obtained asdescribed above. The abscissa indicates the light beam passage positionand the ordinate indicates the output voltage of the integrator 42.

Areas A and C indicates areas in which the light beam does not pass onthe sensor pattern S0 (the light beam passes along while it is deviatedfrom the sensor pattern in the upward or downward direction). Since thesensor pattern S0 does not output a signal in the above areas, theoutput of the integrator 42 is “0”.

An area B is an area in which the light beam passes on the sensorpattern S0. It is understood that the output of the integrator 42 variesin proportion to a variation in the passage position of the light beamwhen the light beam stably passes on or crosses the sensor pattern S0except a case wherein the light beam passes through the edge portion ofthe sensor pattern S0.

Therefore, as described before, the main control section 51 can roughlydetect the position of the sensor pattern S0 on which the light beampasses by reading the result of A/D-conversion of the output of theintegrator 42.

As described above, the main control section 51 can detect the passageposition of the light beam, but in order to enhance the precision ofdetection, it is required for the A/D converter 43 to have highresolution.

For example, a case wherein the distance of the area B is 2048 μm andthe potential difference between Vu and Vd of FIG. 7 is A/D-converted byan 8-bit A/D converter is assumed. In this case, the resolution(precision of detection) becomes 8 μm (=2048 μm/256). If a 12-bit A/Dconverter is used in order to further enhance the precision ofdetection, the resolution (precision of detection) is enhanced to 0.5 μm(=2048 μm/4096). However, if the 12-bit A/D converter is used, the costbecomes extremely high.

For example, if a case wherein it is desired to detect only the passagepositions P1, P2, P3 with high precision is considered, it is notefficient to detect the whole area (2048 μm) of the area B with highresolution.

Therefore, a method for detecting the passage position of the light beamwith high resolution only in a necessary region is explained withreference to FIG. 8.

First, the main control section 51 outputs digital data corresponding tothe voltage V3 to the D/A converter 61 shown in FIG. 5. Since the outputof the D/A converter 61 is input to the inverting input terminal of thedifferential amplifier 60, the differential amplifier 60 outputs avoltage obtained by subtracting the output voltage of the D/A converter61 from the output of the sensor pattern S0.

When the light beam passes through the passage position P3, the outputof the integrator 42 becomes “0”. In other words, the main controlsection 51 outputs a value which causes the output of the integrator 42to become “0” when the light beam passes through the passage position P3to the D/A converter 61.

When the light beam passes through the passage position P1, the outputV1 is lowered by V3 and becomes V1′. That is, the output of theintegrator 42 is shifted downwardly (towards the low voltage side) bythe voltage V3 as shown by (B). Next, the main control section 51 raisesthe amplification factor of the differential amplifier 60. For example,it raises the amplification factor to such a value that the outputvoltage thereof in the passage position P1 will become Vu as shown by(C).

Thus, the voltage variation (range) when the passage position of thelight beam is changed from P1 to P3 can be made large and it becomespossible to enhance the detection resolution (precision) withoutenhancing the resolution of the A/D converter 43.

The principle of detecting the passage position of the light beam in awide range and the principle of enhancing the precision of detection areexplained.

Next, an example in which the conventional light beam passage positiondetecting method is improved by use of the above principles isexplained. FIG. 9 shows an example obtained by improving the sensorpattern of the light beam detecting device 38 disclosed in Japanese Pat.Appln. KOKAI Publication No. 10-76704 by use of the principle of thisinvention explained so far.

That is, the light beam detecting device 38 includes two sensor patternsS1, S2 which are long in the vertical direction, seven sensor patternsSA to SG disposed between the two sensor patterns S1 and S2 and aholding base plate 38 a for integrally holding the sensor patterns S1,S2, SA to SG. For example, the sensor patterns S1, S2, SA to SG areformed of photodiodes.

The sensor pattern S1 is a pattern for detecting passage of the lightbeam to generate a reset signal (integration starting signal) of theintegrator 42 and the sensor pattern S2 is a pattern for detectingpassage of the light beam to generate a conversion starting signal ofthe A/D converter 43. The sensor patterns SA to SG are patterns fordetecting the passage position of the light beam.

As shown in FIG. 9, the sensor patterns S1, S2 are formed to be long inthe sub-scanning direction of the light beam (in a directionperpendicular to the main scanning direction) so that the light beams ato d scanned by the polygon mirror 35 will cross the sensor patternwithout fail irrespective of the position of the galvanomirror 33 b to33 d. For example, in this example, the widths W1, W3 of the sensorpatterns S1, S2 in the main scanning direction of the light beam are 200μm and the length W4 thereof in the sub-scanning direction of the lightbeam is 200 μm.

As shown in FIG. 9, the sensor patterns SA to SG are disposed in alaminated form in the sub-scanning direction between the sensor patternsS1 and S2 and the total length thereof is set to the same as the lengthW4 of the sensor patterns S1, S2. The width W2 of the sensor patterns SAto SG in the main scanning direction is set to 600 μm, for example.

The sensor pattern SA which lies in the upper portion in the drawing isformed in a trapezoidal form in which the size thereof in the mainscanning direction is large in the upper portion and becomes smaller ina portion nearer to the sensor central portion. On the other hand, thesensor pattern SG which lies in the lower portion in the drawing isformed in a trapezoidal form in which the size thereof in the mainscanning direction is smaller in a portion nearer to the sensor centralportion and becomes larger in a portion nearer to the lower side.

With the above structure, in a range in which the sensor patterns detectthe passage of the light beam, the signal outputting time of the sensorpattern SA becomes shorter as the passage position of the light beambecomes lower and the signal outputting time of the sensor pattern SGbecomes longer as the passage position of the light beam becomes lower.

Therefore, even if the passage position of the light beam is greatlydeviated from the detection range of the sensor patterns SB to SF, thedegree of deviation can be easily detected.

Next, an example in which the principle of this invention is applied todetection of inclination of the light beam detecting device 38 withrespect to the main scanning direction of the light beam is explained.

FIG. 10 shows an example obtained by improving the sensor pattern of thelight beam detecting device 38 disclosed in Japanese Pat. Appln. KOKAIPublication No. 9-314901 by use of the principle of this inventionexplained so far. That is, the light beam detecting device 38 includessensor patterns S7 a, S7 b, S1, S3, SH, SA, SB1 to SF1, SB2 to SF2, SG,S4, S5, S6, S8 a, S8 b which are sequentially disposed from the left inthe drawing on a holding base plate 38 a.

The sensor patterns SA, SB1 to SF1, SB2 to SF2, SG are patterns fordetecting the passage position of the light beam, the sensor patternsSB1 to SF1 are used for detection with first resolution (for example,600 dpi) and the sensor patterns SB2 to SF2 are used for detection withsecond resolution (for example, 400 dpi).

The sensor pattern SH is a pattern for detecting the power of the lightbeam. The sensor patterns S4, S5, S6 are patterns for detecting passagetiming of the light beam. The sensor pattern S6 also has a function ofthe sensor pattern S2.

The sensor patterns S7 a, S7 b, S8 a, S8 b are patterns for detectingthe inclination. The sensor patterns S7 a, S7 b and S8 a, S8 b arearranged in the upper and lower positions to make pairs and the centersbetween the sensor patterns S7 a and S7 b and the sensor patterns S8 aand S8 b are set in alignment with the centers of the other sensorpatterns such as the sensor patterns S1, S3.

The principle of this invention is applied to the sensor patterns S7 a,S7 b, S8 a, S8 b and the upper and lower sensor patterns S7 a and S7 band the upper and lower sensor patterns S8 a and S8 b which make pairsare respectively formed in inverted tapered forms.

With the above structure, by comparing the outputs of the sensorpatterns S7 a and S7 b and comparing the outputs of the sensor patternsS8 a and S8 b, the mounting inclination of the light beam detectingdevice 38 with respect to the scanning direction of the light beam canbe detected.

With the above structure, since the outputs of the sensor patterns S7 a,S7 b, S8 a, S8 b are continuously changed with a variation in thepassage position of the light beam in a wide range, the inclination canbe detected in a wide range.

FIG. 11 shows an example in which the sensitivity to the inclination israised in comparison with the case of FIG. 10. The upper and lowersensor patterns S7 a and S7 b and the upper and lower sensor patterns S8a and S8 b which make pairs are respectively formed in inverted taperedforms, but the range of the tapered portion is narrower than in the caseof FIG. 10 (the inclination of the tapered portion is steeper) and thesensitivity for detection of inclination is raised accordingly. In thenormal mounting adjustment, the sufficiently wide range and highsensitivity can be attained with the above structure.

The inclination detecting sensor patterns shown in FIGS. 10, 11 can beused not only for detection of inclination but also for detection of thepassage position of the light beam in the sub-scanning direction asexplained before with reference to FIG. 9. In FIGS. 10, 11, the sensorpattern for outputting the signal pulse for starting A/D conversion andthe sensor pattern for resetting the integrator as explained before arenot shown. FIG. 12 shows an example of the light beam detecting device38 including sensor patterns having the above functions. The light beamdetecting device 38 is explained below.

Two pulse signals of different timings are output when the light beampasses on the patterns of A and B of FIG. 12. A reset signal(corresponding to the pulse signal from the sensor pattern S1) for theintegrator is created based on the two pulse signals. That is, a pulsesignal which is defined by the rising edge of an output of the sensorpattern A and the falling edge of an output of the sensor pattern B iscreated by a logic circuit and input to the integrator as a resetsignal.

The reason why the reset signal is thus created by use of the two sensorpatterns is that resetting of the integrator requires a relatively longtime (which is approximately equal to time during which the light beampasses along between A and B). Another reason is that the rise of thesignal output of the sensor pattern is generally steep and the fallthereof is gentle and it is desirable to use the rise timing of theoutput of the sensor pattern if the precise timing is obtained.

The sensor pattern E is a pattern for outputting an A/D conversion starttiming signal (corresponding to the output of the sensor pattern S2).Therefore, the sensor patterns A, B correspond to the sensor pattern S1whose principle is explained before and the sensor pattern E correspondsto the sensor pattern S2. Further, the sensor patterns C, D correspondto the sensor pattern S0 whose principle is explained before and thesensor patterns S7 a, S7 b shown in FIGS. 10, 11.

Likewise, the sensor patterns K, M correspond to the sensor patterns A,B, that is, the sensor pattern S1 whose principle is explained beforeand the sensor pattern P corresponds to the sensor pattern S2. Further,the sensor patterns O, N correspond to the sensor pattern S0 whoseprinciple is explained before and the sensor patterns S8 a, S8 b shownin FIGS. 10, 11.

Likewise, in a case where the power detection is effected, the sensorpatterns E, K correspond to the sensor patterns A, B, that is, thesensor pattern S1 whose principle is explained before and the sensorpattern M corresponds to the sensor pattern S2. Further, the sensorpattern L for detecting the power corresponds to the sensor pattern S0whose principle is explained before.

FIG. 13 is a diagram for illustrating control of the light beam passageposition when the light beam detecting device shown in FIG. 12 is usedand a portion associated with light beam control is extracted from theblock diagram of FIG. 3 and shown in detail.

As described before, if the light beam detecting device 38 shown in FIG.12 is used, four functions, that is, an inclination detecting functionin a wide range, rough light beam passage position detecting function,precise light beam passage position detecting function and powerdetecting function can be realized.

That is, the sensor patterns C, D, N, O are used for the inclinationdetecting function in a wide range, the sensor patterns C, D or thesensor patterns N, O are used for the rough light beam passage positiondetecting function, the sensor patterns F, G, H, I, J are used for theprecise light beam passage position detecting function and the sensorpattern L is used for the power detecting function. The outputs of thesensor patterns C, D, N, O, F, G, H, I, J, L are respectively amplifiedby amplifiers 63 c, 63 d, 63 n, 630, 63 f, 63 g, 63 h, 63 i, 63 j, 63 land input to a selection circuit (analog switch) 41.

The amplification factors of the respective amplifiers 63 c, 63 d, 63 n,630, 63 f, 63 g, 63 h, 63 i, 63 j, 63 l are set by the main controlsection 51. The selection circuit 41 selects a signal input to theintegrator 42 according to the sensor selection signal from the maincontrol section 51 and the selected signal is input to and integrated bythe integrator.

In FIG. 13, the D/A converter 61 shown in FIG. 5 is not shown. As willbe described later, this is because it is not necessary to electricallyswitch the precision since the patterns F, G, H, I, J for preciselydetecting the light beam passage position and the patterns C, D, N, Ofor detecting the light beam passage position in a wide range areseparately provided as the sensor patterns.

In the block diagram of FIG. 13, it is necessary to change the timingsof the reset signal (integration start signal) and the A/D conversionstart signal according to the sensor pattern to be detected since theintegrator 42 and A/D converter 43 are commonly used. This is madepossible by use of a reset signal creating circuit 64 and A/D conversionstart signal creating circuit 65. The outputs of the sensor patterns A,B, E, K, M are input to the reset signal creating circuit 64. Asexplained before, the reset signal for the integrator 42 is created byuse of two signals among the above outputs and input to the integrator42. A method for creating the reset signal by combining what type ofsignals is determined by the main control section 51.

Further, the outputs of the sensor patterns E, K, M, P are input to theA/D conversion start signal creating circuit 65 and the main controlsection 51 can select an adequate signal.

That is, the main control section 51 can selectively determine theoutputs of the sensor patterns which are combined to create the resetsignal and the output of the sensor pattern which is used as theconversion start signal for the circuits 64, 65 according to the sensorpattern to be detected.

The sensor patterns to be detected and settings for the circuits 64, 65are indicated in the following table 1. TABLE 1 reset (leading to-be-edge- A/D detected trailing conversion No. detection items objects edge)Start 1 inclination, C, D A-B E passage position 2 Passage F, G, I, JB-E K position 3 Power L E-K M 4 Inclination, N, O K-M P passagepositionThus, the main control section 51 adequately selects the to-be-detectedsensor pattern, effects the integrating operation and A/D convertingoperation in the optimum state and can fetch information as digitaldata.

The main control section 51 realizes the four functions of detecting theinclination of the light beam detecting device 38 and roughly detectingthe light beam passage position based on both of the detection items ofNo. 1 and No. 4 in the table 1, precisely detecting the light beampassage position based on the detection items of No. 1 and No. 2 in thetable 1, and detecting the power based on the detection items of No. 1and No. 3 in the table 1.

The operations of the amplifiers 63 c, 63 d, 63 n, 63 o, 63 f, 63 g, 63h, 63 i, 63 j, 63 l, integrator 42 and A/D converter 43 are the same asexplained with reference to FIG. 5 and the explanation for theoperations is omitted here.

In FIG. 13, four sets of galvanomirrors and galvanomirror drivingcircuits for driving the galvanomirrors are shown and the number of setsis larger than the number of sets of galvanomirrors and galvanomirrordriving circuits shown in FIGS. 2 and 5 by one.

This is because a case wherein the light beam passage position detectingmethod of this invention is applied to the conventional light beampassage position detecting device and all of the light beam passagepositions are required to be controlled by the control method describedin Japanese Pat. Appln. KOKAI Publication No. 10-76704, for example.

Therefore, in this example, it is impossible to attain one of theobjects of this invention, that is, to suppress to a least sufficientnumber the number of actuators for permitting the relative passageposition of the light beam to be detected in a wide range andcontrolling the passage position of the light beam (for example, tosuppress the four actuators in the conventional case to three). However,as described above, the inclination detecting range advantageous foradjustment of the mounting inclination of the light beam detectingdevice can be made wider in comparison with the conventional case andthe rough light beam passage position detecting function suitable forrough adjustment of the light beam passage position can be attained. Theconstruction of the optical system unit (corresponding to FIG. 2) usedin this case is described in detail in Japanese Pat. Appln. KOKAIPublication No. 10-76704, for example, and the explanation therefore isomitted here.

Next, the outputs of the sensor patterns relating to the detection itemsof No. 1, No. 2, No. 4 in the above table 1 are explained in detailbelow.

FIG. 14 shows the positional relation between the sensor patterns C (N),D (O) and the sensor patterns F, G, H, I, J and the relation between thepositions and outputs of the sensor patterns. As shown in FIG. 14, theoutputs of the tapered sensor patterns C (N), D (O) gradually vary witha variation in the light beam passage position. On the other hand, theoutputs of the sensor patterns F, G, H, I, J abruptly vary with a slightvariation in the light beam passage position.

Therefore, when the light beam passage position is greatly deviated fromthe positions of the sensor patterns F, G, H, I, J, the controloperation can be efficiently effected by determining the light beampassage position based on the output signals of the tapered sensorpatterns C (N), D (O) to control the galvanomirrors.

That is, the light beam detecting device 38 shown here has both of thelight beam passage position detecting function of wide range for roughadjustment and the precise light beam passage position detectingfunction for fine adjustment in the light beam passage position controloperation.

FIG. 15 shows a light beam detecting device 38 having sensor patternsusing the principle of this invention for detecting the light beampassage position and sensor patterns disclosed in Japanese Pat. Appln.KOKAI Publication No. 10-76704 which are integrally formed.

That is, the sensor patterns S1, S0, S2 are the sensor patterns of thisinvention which are explained so far and sensor patterns S15, S16, S17and sensor patterns S18, S19, S20 are the same as the sensor patternarray disclosed in Japanese Pat. Appln. KOKAI Publication No. 10-76704.

The arrangement of the sensor patterns S15, S16, S17 and sensor patternsS18, S19, S20 is briefly explained. For example, as shown in FIG. 16A,the sensor patterns S15, S16, S17 are arranged such that the sizethereof in the main scanning direction of the light beam is 600 μm, thesize thereof in the sub-scanning direction of the light beam is 32.2 μmand they are arranged at a regular interval of 10 μm in the sub-scanningdirection. Therefore, the pitch between the gap center between thesensor patterns S15 and S16 and the gap center between the sensorpatterns S16 and S17 is 42.3 μm.

For example, as shown in FIG. 16B, the size of each of the sensorpatterns S18, S19, S20 is set to 600 μm in the main scanning directionand to 53.5 μm in the sub-scanning direction, and like the sensorpatterns S15, S16, S17, they are arranged at a regular interval of 10 μmin the sub-scanning direction. Therefore, the pitch between the gapcenter between the sensor patterns S18 and S19 and the gap centerbetween the sensor patterns S19 and S20 is 63.5 μm.

The light beam passage position control operation using the sensorpatterns is disclosed in detail in Japanese Pat. Appln. KOKAIPublication No. 10-76704, but the feature thereof is briefly explainedhere.

For example, the sensor patterns S15, S16, S17 are sensor patterns forsetting the light beam passage pitch (in the sub-scanning direction) tocorrespond to the resolution (first resolution) of 600 dpi. The intervalbetween the two light beam passage positions is set to 42.3 μm, that is,it is set to correspond to the resolution of 600 dpi by driving onelight beam into a portion (gap) between the sensor patterns S15 and S16and driving the other light beam into a portion (gap) between the sensorpatterns S16 and S17. On the other hand, for example, the sensorpatterns S18, S19, S20 are sensor patterns for setting the light beampassage pitch to correspond to the resolution (second resolution) of 400dpi. The interval between the two light beam passage positions is set to63.5 μm, that is, it is set to correspond to the resolution of 400 dpiby driving one light beam into a portion (gap) between the sensorpatterns S18 and S19 and driving the other light beam into a portion(gap) between the sensor patterns S19 and S20.

Thus, since the array pitch of the sensor patterns is set equal to thelight beam passage position pitch necessary for image formation, thelight beam can be driven into a desired passage location with highprecision.

However, the conventional light beam detecting device to which the abovesystem is applied has a defect that the number of sensor patterns isincreased when a large number of light beams to be controlled are usedor the light beam passage pitch is set to correspond to a plurality ofresolutions. Further, since it is necessary to control each light beaminto a specified passage position between the sensor patterns, it isrequired to provide actuators (galvanomirrors) for changing the passagepositions of the respective light beams.

The relative passage position can be detected by use of the light beampassage position detecting method of this invention explained so far ifthe light beam passes on the sensor pattern. Therefore, it is possibleto reduce the number of actuators (galvanomirrors) by fixing the passageposition of one light beam and controlling the passage position orpositions of the other light beam or beams with the former light beamused as a reference.

However, when the light beam passage position is determined by use ofonly the tapered sensor pattern S0, the relative position of each lightbeam can be detected, but the absolute position thereof cannot bedetected.

Therefore, in this invention, the sensor patterns such as the sensorpatterns S15, S16, Sl7 and the sensor patterns S18, S19, S20 fordetecting the absolute passage position of the light beam and the sensorpattern S0 for detecting the relative passage position of the light beamas explained before are integrally formed in the light beam detectingdevice so that both of the absolute passage position of the light beamand the relative positional relation can be detected.

That is, it is possible to use the sensor patterns S15, S16, S17 and thesensor patterns S18, S19, S20 as the absolute distance reference on thesensor and correct the result of detection by the sensor pattern S0.

Next, a method for permitting both of the absolute passage position ofthe light beam and the relative positional relation to be detected byuse of the light beam detecting device 38 shown in FIG. 15 is explainedwith reference to the flowchart shown in FIG. 17.

First, the actuator is controlled so as to permit the light beam to passthrough between the sensor patterns S15 and S16 by use of the light beamwhose passage position can be changed by use of the actuator such as thegalvanomirror (S171). The method is described in detail in Japanese Pat.Appln. KOKAI Publication No. 10-76704 and the explanation therefore isomitted.

A value output from the sensor pattern S0 is measured while the lightbeam passes through between the sensor patterns S15 and S16 and it isstored as A/D(S15-S16) (S172).

Next, the actuator is controlled so as to permit the beam light to passthrough between the sensor patterns S16 and S17 by use of the light beamwhose passage position can be changed by use of the actuator such as thegalvanomirror (S173).

A value output from the sensor pattern S0 is measured while the lightbeam passes through between the sensor patterns S16 and S17 and it isstored as A/D(S16-S17) (S174).

A difference between the output of the sensor pattern S0 while the lightbeam passes through between the sensor patterns S15 and S16 and theoutput of the sensor pattern S0 while the light beam passes throughbetween the sensor patterns S16 and S17 is calculated and stored asA/D(42.3) (S175).

The thus calculated value of A/D(42.3) indicates a variation amount ofthe output of the sensor pattern S0 when the light beam passage positionis moved by 42.3 μm in the sub-scanning direction. Therefore, if theactuators are controlled to adjust the passage positions of the otherlight beams to preset positions based on the above value, the passagepositions thereof can be precisely moved by 42.3 μm in the sub-scanningdirection.

The operation for adjusting the passage positions of the plurality oflight beams is effected after the powers of the plurality of light beamsare uniformly adjusted. This is because variation amounts of the outputsfrom the sensor pattern S0 when the light beam passage positions aremoved by 42.3 μm become different for the respective light beams if thepowers of the light beams are not made uniform.

A case wherein the light beam passage positions are moved by 42.3 μm isexplained, but it is clearly understood that the light beam passagepositions can be precisely moved by 63.5 μm if the sensor patterns S18,S19, S20 are used and the explanation therefore is omitted.

Next another embodiment using the same principle is explained.

Like the case of FIG. 15, FIG. 18 shows a light beam detecting device 38having sensor patterns using the principle of this invention fordetecting the light beam passage position and sensor patterns disclosedin Japanese Pat. Appln. KOKAI Publication No. 10-76704 which areintegrally formed.

That is, sensor patterns S1, S0, S2 are the sensor patterns of thisinvention which are explained so far and sensor patterns S21, S22, S23are the same as the sensor pattern array disclosed in Japanese Pat.Appln. KOKAI Publication No. 10-76704.

The arrangement of the sensor patterns S21, S22, S23 is brieflyexplained. For example, as shown in FIG. 19, the sensor patterns S21,S22, S23 are arranged such that the size thereof in the light beamscanning direction (main scanning direction) is 600 μm, the size thereofin a direction (sub-scanning direction) perpendicular to the light beamscanning direction is 90 μm and they are arranged at a regular intervalof 10 μm in the sub-scanning direction. Therefore, the pitch between thegap center between the sensor patterns S21 and S22 and the gap centerbetween the sensor patterns S22 and S23 is 100 μm.

Next, a method for controlling the passage position of the light beam byuse of the light beam detecting device 38 shown in FIG. 18 is explainedwith reference to the flowchart shown in FIG. 20. In this example, acase wherein an inexpensive 8-bit device is used as the A/D converter 43is explained.

First, the actuator is controlled so as to permit the beam light to passthrough between the sensor patterns S21 and S22 based on outputs of thesensor patterns S2, S4, S21, S22 by use of the light beam whose passageposition can be changed by use of the actuator such as the galvanomirror(S201).

An output value to the D/A converter 61 (refer to FIG. 5) is changed S0as to set a value output from the sensor pattern S0 to “0” while thelight beam passes through between the sensor patterns S21 and S22(S202).

Next, the actuator is controlled so as to permit the beam light to passthrough between the sensor patterns S22 and S23 by use of the light beamwhose passage position can be changed by use of the actuator such as thegalvanomirror (S203).

The amplification factor of the amplifier 60 is set so as to set a valueoutput from the sensor pattern S0 to “250” while the light beam passesthrough between the sensor patterns S22 and S23 (S204). Then, thepassage position of the light beam is controlled by dealing with each(“1”) of the values of the output from the sensor pattern S0 as 0.4 μm(100 μm/250) (S205).

Thus, it becomes possible to set a special relation between the outputof the sensor pattern S0 and a variation in the light beam passageposition by setting the output to the D/A converter 61 and theamplification factor of the amplifier 60 so that the output of thesensor pattern S0 will be set to “0” while the beam light passes throughbetween the sensor patterns S21 and S22 and the output of the sensorpattern S0 will be set to “250” while the beam light passes throughbetween the sensor patterns S22 and S23.

That is, when the passage position of the light beam is changed from aportion between the sensor patterns S21 and S22 to a portion between thesensor patterns S22 and S23, the passage position is changed by 100 μmon the light beam detecting device 38. At this time, since a change inthe output of the sensor pattern S0 is “250”, the variation in the lightbeam passage position for each output value “1” of the sensor pattern S0can be set to 0.4 μm (100 μm/250).

Therefore, the main control section 51 can control the passage positionsof the respective light beams to desired positions by measuring therelative passage positions of the light beams by use of the output ofthe sensor pattern S0 and controlling the actuators by use of the resultof measurement.

In the example in which the 8-bit A/D converter is used as the A/Dconverter 43, the measurement range is 102.4 μm (0.4 μm×256).

Therefore, the to-be-measured light beam may exceed the range of the A/Dconverter 43 to cause a trouble in the measurement in some cases.However, in such a case, the trouble can be coped with by changing thesetting value to the D/A converter 61 to move the measurement range.

Next, still another embodiment using the principle of this invention isexplained.

FIG. 21 shows a light beam detecting device 38 having sensor patternsS4, S30, S31, S32, S33, S34 in addition to the sensor patterns S1, S0,S2 using the principle of this invention for detecting the light beampassage position.

For example, as shown in FIG. 22A, the shape of the sensor pattern S0 isa trapezoidal form in which the length thereof in the sub-scanningdirection is 2048 μm, the length of the long side in the main scanningdirection is 1536 μm and the length of the short side is 512 μm.Therefore, the inclination of the inclined side is “2” (2048/1024).

When the light beam scanned by the polygon mirror passes on the sensorpattern S0, the ratio of a variation (in the main scanning direction) inthe distance by which the light beam travels on the sensor pattern to avariation (in the sub-scanning direction) in the light beam passageposition is 2:1. This is because the inclination of the inclined side ofthe sensor pattern S0 is “2”.

For example, the sensor pattern S34 is formed in a rectangular form inwhich the length of a side in the sub-scanning direction is 2048 μm andthe length (D12) of a side in the main scanning direction is 1024 μm.For example, the sensor pattern S33 is formed in a rectangular form inwhich the length of a side in the sub-scanning direction is 2048 μm andthe length (D11) of a side in the main scanning direction is 1002.85 μm.For example, the sensor pattern S32 is formed in a rectangular form inwhich the length of a side in the sub-scanning direction is 2048 μm andthe length (D10) of a side in the main scanning direction is 992.25 μm.

That is, differences of 31.75 μm and 21.15 μm are set between thelengths of the sensor patterns S32 and S34 and the lengths of the sensorpatterns S33 and S34 in the main scanning direction. The differences areset equal to variations in the traveling distance of the light beam onthe sensor pattern S0 when the position in which the light beam passeson the sensor pattern S0 is moved by 63.5 μm (corresponding to theresolution of 400 dpi) and 42.3 μm (corresponding to the resolution of600 dpi) in the sub-scanning direction.

For example, as shown in FIG. 22B, when the light beam passes throughthe mid portion in the sub-scanning direction, the traveling distance onthe sensor pattern S0 is 1024 μm (=(1536+512)/2). If a case wherein thelight beam passage position is downwardly shifted by 42.3 μm isconsidered, then the light beam traveling distance on the sensor patternS0 is shortened by 21.15 (=42.3/2) μm and becomes 1002.85 μm. Therefore,for example, a value obtained by subtracting the output integratingvalue of the sensor pattern S33 from the output integrating value(reference value) of the sensor pattern S34 becomes equal to a variationamount of the output integrating value of the sensor pattern S0 obtainedwhen the light beam passage position is changed by 42.3 μm. Therefore,the light beam passage position is so controlled that a differencebetween the output integrating value of the sensor pattern S0 by thefirst light beam and the output integrating value of the sensor patternS0 by the second light beam will become equal to a difference betweenthe output integrating values obtained when the light beam passes on thesensor patterns S34 and S33. As a result, the first and second lightbeams scan the light beam detecting device 38 with a distance of 42.3 μmset apart from each other in the sub-scanning direction.

Further, for example, a value obtained by subtracting the outputintegrating value of the sensor pattern S32 from the output integratingvalue (reference value) of the sensor pattern S34 becomes equal to avariation amount of the output integrating value of the sensor patternS0 obtained when the light beam passage position is changed by 63.5 μm.

Therefore, the light beam passage position is so controlled that adifference between the output integrating value of the sensor pattern S0by the reference light beam and the output integrating value of thesensor pattern S0 by the to-be-controlled light beam will become equalto a difference between the output integrating values obtained when thelight beam passes on the sensor patterns S34 and S32. As a result, thereference and to-be-controlled light beams scan the light beam detectingdevice 38 with a distance of 63.5 μm set apart from each other in thesub-scanning direction.

The operation for controlling the light beam passage position by usingthe light beam detecting device 38 of FIG. 21 has been explained above.

Next, a method for controlling the light beam passage position by use ofthe same principle is explained.

FIG. 23 shows a light beam detecting device 38 having sensor patternsS4, S40, S41, S42 in addition to the sensor patterns S1, S0, S2 usingthe principle of this invention for detecting the light beam passageposition.

The principle used for control is the same as that used in the case ofFIG. 21, but it is featured in the difference between the sizes of thesensor patterns S41 and S42 in the main scanning direction. That is, thesize D21 of the sensor pattern S42 in the main scanning direction is1024 μm, for example, and the size D20 of the sensor pattern S41 in themain scanning direction is 974 μm, for example. A difference between thesizes of D21 and D20 is 50 μm. As shown in FIG. 23, the difference isequal to the light beam traveling distance on the sensor pattern S0 whenthe light beam passage position is changed by 100 μm in the mainscanning direction.

Therefore, for example, a value obtained by subtracting the outputintegrating value of the sensor pattern S41 from the output integratingvalue (reference value) of the sensor pattern S42 becomes equal to avariation amount of the output integrating value of the sensor patternS0 obtained when the light beam passage position on the sensor patternS0 is changed by 100 μm. That is, a variation amount of the outputintegrating value of the sensor pattern S0 with respect to a variationof 100 μm of the light beam passage position in the sub-scanningdirection can be obtained. The relation can be used for controlling thelight beam passage position.

Also, in this embodiment, like the control method using the light beamdetecting device 38 shown in FIGS. 18 and 19, the above relation (avariation amount of the output integrating value of the sensor patternS0 obtained when the light beam passage position is changed by 100 μm)can be easily derived by changing the setting value to the D/A converter61 of FIG. 5 and the setting value to the differential amplifier 60.

By using the above relation, the passage position of theto-be-controlled light beam is changed so that the output integratingvalue of the sensor pattern S0 by the reference light beam and theoutput integrating value of the sensor pattern S0 by theto-be-controlled light beam can be set to a preset relation. As aresult, the light beams scan the light beam detecting device 38 with apreset distance therebetween.

As described above, according to this invention, a light beam scanningapparatus can be provided in which the relative and absolute scanningpositions of the light beam can be precisely detected in a wide rangeand the scanning position of the light beam can be controlled to apreset position by use of the least sufficient number of actuators forcontrolling the light beam passage position.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A light beam scanning apparatus comprising: light beam emitting meansfor outputting a plurality of light beams; a beam scanner for reflectingthe light beam output from said light beam emitting means towards ato-be-scanned surface to scan the to-be-scanned surface by use of thelight beams in a main scanning direction; a beam position detectorincluding a first beam sensing section for detecting the light beamsscanned on the to-be-scanned surface by said beam scanner and said beamposition detector generating an output signal whose level iscontinuously changed with a variation in the passage position in asub-scanning direction perpendicular to the main scanning direction ofthe light beam; and a controller for controlling of the light beamsscanned by said beam scanner on the to-be-scanned surface to be in apreset position based on the level of the output signal of said beamposition detector.
 2. The light beam scanning apparatus according toclaim 1, which further comprises level shift means for shifting thelevel of the output signal of said beam position detector, saidcontroller controlling the light beams scanned by said beam scanner onthe to-be-scanned surface to be in the preset position based on thelevel of the output signal of said beam position detector shifted bysaid level shift means.
 3. The apparatus according to claim 1, whichfurther comprises passage detecting means including a second beamsensing section arranged on the upstream side in the main scanningdirection of the light beam with respect to the first beam sensingsection of said first beam position detector, said passage detectingmeans detecting the passage of the light beam scanned by said beamscanner and generating a timing signal; level shift means for shifting alevel of an output signal of said beam position detector; andintegration means for starting integration of the output signal of saidbeam position detector shifted by said level shift means, theintegration being started in response to a timing signal supplied bysaid passage detecting means; said controller controlling the lightbeams to be in the preset position based on the output of saidintegration means.
 4. The apparatus according to claim 3, which furthercomprises: second passage detecting means including a third beam sensingsection arranged on the downstream side in the main scanning directionof the light beam with respect to the first beam sensing section of saidbeam position detector, said second passage detecting means detectingthe passage of the light beam scanned by said beam scanner andgenerating a second timing signal; converting means for converting ofthe output signal, generated by the integration means into a digitalsignal in response to the second timing signal from the second passagesensing means; and in which said controller controls each passageposition of the light beams to said preset position based on the digitalsignal converted by said converting means.
 5. An image forming apparatuscomprising: light beam emitting means for outputting a plurality oflight beams according to an image data; a beam scanner for reflectingthe light beam output from said light beam emitting means towards animage forming medium to scan the image forming medium by use of thelight beam in a main scanning direction for forming an image on theimage forming medium according to the image data; a beam positiondetector for detecting the light beams scanned on the image formingmedium by said beam scanner and generating an output signal whose levelis continuously changed with a variation in the passage position in asub-scanning direction perpendicular to the main scanning direction ofthe light beam: and a controller for controlling the light beams scannedby said beam scanner on the image forming medium to be in a presetposition based on the level of the output signal generated by said beamposition detector.
 6. A light beam scanning apparatus comprising: lightbeam emitting means for outputting a plurality of light beams; a beamscanner for reflecting the light beam output from said light beamemitting means towards a to-be-scanned surface to scan the to-be-scannedsurface by use of the light beams in a main scanning direction; a beamposition detector including a photo-diode for detecting the light beamsscanned on the to-be-scanned surface by said beam scanner, saidphoto-diode having a width which is defined in the main scanningdirection and which varies depending upon a position determined in asub-scan direction crossing the main scanning direction, and said beamposition detector generating an output signal whose level iscontinuously changed with a variation in the passage position of thelight beam in the sub-scanning direction; and a controller forcontrolling each position of the light beams scanned by said beamscanner on the to-be-scanned surface to a preset position based on thelevel of the output signal of said beam position detector.